Both the width and height are set to 840.
This may have helped me track the issue down a bit further. It seems that the issue is only happening for posts whose image is exactly that size. If I change that width and height to almost any other value, the issue goes away. Its only when its set to 840 or 839 px that I’ve found the issue is there.
I’m not sure if that helps clarify a reason / solution on your end.